The present invention relates to strengthening of welding seams and more particularly the present invention relates to strengthening of the seam of a longitudinally welded thin-walled, nonferrous, metal tube.
IT WAS FOUND THAT LONGITUDINALLY SEAM WELDED TUBES PARTICULARLY OF THE VARIETY MADE OF NONFERROUS METAL SUCH AS COPPER, ALUMINUM OR AN ALLOY THEREOF, HAVE WELDING SEAMS WITH A COARSER METAL GRAIN TEXTURE THAN THE TEXTURE OF THE TUBE MATERIAL ITSELF. Such a coarser texture is undesirable particularly because it represents a weakenss in the material and renders the tube more prone to corrosion. Also the tube is more likely to leak or to develop leaks and fissures which, after some period of time, may turn into leaking defects. Therefore, this coarseness in texture has to be eliminated.
A particular method is known for the production of longitudinally seam welded tubes in which a longitudinally paid, metal strip is continuously deformed into a split tube, and the adjoining edges are subsequently welded. After cooling, the welding seam is deformed by means of a pressure roll working from the outside and against a stationary mandrel in the interior of the tube. Such a method is, for example, disclosed in U.S. Pat. No. 3,650,456. This method has the drawback that the underside of the seam, being on the inside of the tube, slides on the stationary mandrel. Of course, the mandrel can be lubricated, but sooner or later some friction and wear will be encountered. Aside from interferring with the production it was found that this method is actually suitable only for tubes which have relatively thick walls. For thin walls, it is not suitable, because sliding friction tends to immediately introduce weak spots in operation or abrasion.